The present invention relates to a circuit for protection against overvoltages.
Buses used to transfer data between multiple users via a common transmission path are known in the art. The single-wire bus, or 1-wire bus, is characterized in that a serial bidirectional data transmission and a power supply of connected devices or components can be carried out via a single data line, i.e., the input signals, output signals, and a power supply voltage can be transmitted via one and the same data line. A ground line is required in addition to the data line.
A master device and at least one slave device are provided for each 1-wire bus. A controller such as a microcontroller or PC with an adapter can be used as a master device, which is equipped with a passive or active pull-up resistor. However, a special master device, such as the DS2482-100 or the DS2482-800 from the company Maxim Integrated Products, Inc., can also be used. The slave devices may be e.g. sensors, such as temperature or humidity sensors, memory components, analog-digital converters, digital potentiometers, time recording and real-time clocks, battery protectors, selectors and monitors. The operating voltage of the components is generally between 2.8 V and 6 V. Many components operate with an operating voltage that should be between 2.8 V and 5.5 V. However, only 3.0 V to 3.7 V are approved for some components.
A special feature of the 1-Wire devices is that the slave devices can be supplied with a parasitic voltage from the data line. When the communication is inactive, i.e., in the idle state or rest state, the data line is at a high potential of, for example, 5 V, and charges a respective storage capacitor integrated in each slave device. The data line is hereby pulled up to the high potential with the aid of a pull-up resistor. During operating phases where the data line is at a low potential, each slave device is powered from its storage capacitor.
A data line of a 1-wire bus operates bidirectionally at a potential of, for example, 3.3 V and is provided with an internal pull-up resistor via a master device. Accidental application of a voltage to the input port or terminal of the master device (the data line) of greater than about 0.5 volts above its supply voltage, i.e. of a voltage greater than about Vcc+0.5 V, results in destruction of the master device, which must therefore be protected from such an overvoltage. The same applies to a circuit or another electronic functional unit connected to the data line. This problem occurs not only with a 1-Wire bus, but also with other bidirectional bus systems, such as an I2C-bus or an RS485 bus in half-duplex mode.
Previously known devices for protection against positive and negative overvoltages, such as the protection circuit described in the document DE 199 16 685 A1, frequently operate with a limiting or ballast resistor. Such protection circuits are not suitable for a 1-wire bus, because a corresponding voltage would drop across the ballast resistor, so that the slave devices connected to the 1-wire bus would receive an inadequately low supply voltage due to the voltage drop across the ballast resistor.
A similar situation occurs with protection circuits having a Zener diode or Z-diode, across which at a voltage drop would also occur and which moreover would also not protect against negative overvoltages. Conversely, protection circuits with a bidirectional suppressor diode would protect only against overvoltage transients, but not against prolonged overvoltages. Protection circuits realized with a field effect transistor are integrated in line carrying a positive voltage or negative voltage, whereas protection circuits realized with two field effect transistors are integrated in a line carrying a positive voltage. Bi-directionality could therefore be implemented in protection circuits having field effect transistors either not at all or only with directional control, wherein a bidirectional protection circuit with direction control is not suitable for use in conjunction with a 1-wire bus.
Other prior art protection circuits are for similar reasons also not suitable for a 1-wire bus. For example, in protection devices with a varistor, a corresponding voltage would drop across the varistor. Protection circuits with a thyristor, which triggers a fuse in the event of an active short-circuit of a supply voltage, would not operate without a supply voltage and would thus not protect against negative overvoltages.